The present invention relates to a gear assembly. In particular, the present invention relates to a gear whose teeth flanks are shaped to provide a gear assembly having an increased contact ratio.
The contact ratio for a pair of gear elements provides an indication of the maximum number of gear teeth of the pair of gear elements which are contact at any given time. Since the contact ratio is related to the maximum torque which can be transmitted between the gear elements, it has been desirable to produce increase the contact ratio of mating gear elements.
Conventionally, the solution has been to alter the angle of the gear teeth so that the gear teeth are no longer parallel to the rotational axis of the gear. However, this solution reduces the contact area between mating gear teeth to a point, thereby minimizing the impact of the alteration on torque capacity. Accordingly, other attempts have been made to produce gear elements having an increased contact ratio.
For example, Rennerfelt (U.S. Pat. No. 5,030,184) teaches an eccentric gear comprising a fixed spur gear having internal teeth, an eccentric cam secured to the input shaft of the eccentric gear, and an eccentric gear wheel rotatably coupled to the eccentric cam and having teeth engaging the internal teeth of the fixed spur gear. The teeth of the eccentric gear wheel and the teeth of the fixed spur gear are corrected by profile shifting to prevent collisions between the teeth and by stubbing to prevent collisions between the lands. The patentee states that the described profile shifting and stubbing increases the number of teeth in mesh. However, the patentee also acknowledges that, in theory, only one tooth in each loading direction will simultaneously be in mesh and that, in practice, one can achieve a greater number of teeth in mesh only by relying on the resilient nature of the teeth. However, such a practice can only increase the stress on each tooth. Therefore, there remains a need for gear box having gears manufactured with teeth shaped to increase the contact ratio between congruent teeth flanks so as to reduce contact stress on the teeth bearing surfaces.
It is an object of the present invention to provide a gear assembly and a pinion whose gear teeth are shaped to provide an increased contact ratio between congruent teeth flanks. It is also an object of the present invention to provide a method for forming said congruent teeth flanks.
The gear assembly, according to the invention, comprises a first gear including first set of teeth and a first addendum circle, and a second gear having a second set of teeth meshing with the first set of teeth. The shape of the addendum flank of each first tooth and the shape of the addendum flank of each corresponding second tooth are defined by a curve of congruency which passes between the addendum circles and extends between the point of intersection of the addendum circles and the pitch point of the gears.
The pinion, according to the invention, includes a set of gear teeth configured for meshing with gear teeth of a gear, with the pinion and gear each having a respective addendum circle. The shape of the addendum flank of at least one of the gear teeth of the pinion is defined by a curve of congruency which passes between the addendum circles and extends between the point of intersection of the addendum circles and the pitch point of the pinion and the gear.
The method, according to the invention, forms teeth flanks for a pinion congruent with teeth flanks of a gear, with the pinion and the gear each having a respective addendum circle and a respective pitch circle. The method includes the steps of (1) defining a curve congruent with the addendum circles at least between a point of intersection of the addendum circles and a point of intersection of the pitch circles; (2) for each of a plurality of points on the curve, determining polar co-ordinates for each respective point relative to a centre of rotation of the pinion; and (3) defining a shape of an addendum of each said tooth flank in accordance with the determined polar co-ordinates.